In recent years, surface modification of materials has been widely challenged in order to enhance an added value of the materials in a variety of fields. Particularly, fine particles which have been prepared by processing a material to a nanoscale (hereinafter, referred to as “nanoparticles”) have a very large specific surface area, the surface properties of which can be readily modified, and thus their affinity, adhesive property in living bodies, etc. can be controlled; therefore, it is anticipated that the fine particles will find application in catalysts, color materials, electronic materials, optical materials, medical supplies, cosmetic products, etc.
Among such nanoparticles, gold nanoparticles have a characteristic property of producing a bright pink color by surface plasmon absorption when in a dispersed state and changing the color hue to purple when in an aggregated state. Therefore, gold nanoparticles have been actively challenged for their applications in biotechnology fields through stably dispersing the gold nanoparticles and functionalizing the surface thereof.
A thiolated poly(ethylene glycol) (PEG-SH) has heretofore been reported by Murray et al. (see Non-Patent Document 1) to be a dispersant capable of stably dispersing gold nanoparticles. On the other hand, the present inventors have developed and reported a poly(ethylene glycol) (hetero PEG) having a thiol group with higher coordination ability to surface of gold at one terminus and a functional group for introducing a ligand at another terminus (see Non-Patent Document 2). If various ligands such as antibodies, oligo DNAs, and sugars (sugar chain) are introduced into the hetero PEG, the gold nanoparticles are provided with the ligands on their dispersed surface and thus will be utilized as practicable diagnosing or analyzing nanoparticles.
Incidentally, it is necessary for the purpose of stably dispersing nanoparticles as an object to be modified to a satisfactory level that the above-mentioned thiols coat the surface of particles more densely than polymers functional as a multidentate ligand since the thiols are a monodentate ligand. Therefore, it will also be preferred if multipoint adsorption and stabilization can be achieved by grafting a polymerizable monomer having a thiol group. However, the high reactivity of the thiol group easily leads to deprotection and cross-linking even if the polymerizable monomer has a terminal protective group, therefore, it is difficult to graft polymerizable monomers having a thiol group.
Then, the present inventors have investigated to find that the graft copolymer between the polymerizable monomer (A) represented by general formula (I) and the polymerizable monomer (B) represented by general formula (II) is useful as a polymeric molecular structure of a surface modifier (see Non-Patent Document 3).

However, in accordance with the above-mentioned finding, the surface of gold nanoparticles could not be functionalized even if the gold nanoparticles can be more stably dispersed since a pyridyl group undergoes multipoint coordination on the surface of the gold nanoparticles.    [Non-Patent Document 1] W. Peter Wuelfing, S. M. G., Deon T. Miles, Royce W. Murray; Journal of American Chemical Society; 1998; 120(48); pp. 12696-12697.    [Non-Patent Document 2] Hidenori Otsuka, Y. A., Yukio Nagasaki, Kazunori Kataoka; Journal of American Chemical Society; 2001; 123(34); pp. 8226-8230.    [Non-Patent Document 3] Tomomi Satomi, Koji Ueno, Hisatoshi Kobayashi, Junzo Tanaka, Yoshinori Mitamura, Tetsuya Tateishi, and Hidenori Otsuka, Journal of Nanoscience and Nanotechnology, 2006; 6, pp. 1792-1796.